The present invention relates to a pressure control unit with an inlet chamber which may be connected to a master cylinder, an outlet chamber which may be connected to a slave cylinder, a valve interconnected between the two chambers and closing upon a predetermined switching pressure, and a check valve connected in parallel therewith as far as operation is concerned, a pressure reduction in the inlet chamber relative to the outlet chamber causing the check valve to open against a closing force.
Such a pressure control unit is described in the U.S. Pat. No. 3,088,285. In the known device the annular seal has two parallel end faces, a cylindrical inside circumferential surface, and a slightly arched outside circumferential surface. The outside circumferential surface serves to override the end of the by-pass channel designed as bore, annular gap, or the like. The size of the inside circumferential surface slightly exceeds that of the portion of the piston associated with the annular seal. The annular seal is held between two flanges of the piston, the distance between them slightly exceeding the height of the annular seal. The flange adjacent to the inlet chamber has a bore leading to the inside diameter. During normal operation, i.e. with the inlet pressure predominating, the annular seal will be pressed in a sealed relation against the flange adjacent to the outlet chamber. Thus, any communication between the inlet and outlet chambers is only possible via the by-pass channel. However, if the outlet pressure is predominating, the annular seal will be displaced axially. Thus, a return channel is formed which will extend along the one end face and the inside circumferential surface of the annular seal. Very close manufacturing tolerances have to be observed in this design since even slight inaccuracies will be sufficient to keep the return channel permanently closed, or to have the outlet surface abut the circumference of the cylinder chamber with excessive force. A reliable mode of operation of the known pressure control unit is likewise not safeguarded due to the fact that the elastic annular seal is deformed by the excessive stress forces, and, thus, tolerances ensuing in the closing distance. Moreover, wear has a limiting effect on its life. The annular seal does not only perform a relative movement with respect to the housing, but also a relative movement with respect to the piston. The thus ensuing manifestations of wear will in course of time lead to changes in the dimensions which will cause an unreliable sealing action.